The present invention relates generally to methods and apparatus for the handling of high activity radioactive sources in the treatment of cancerous tissue.
The use of radioactive material in the treatment of cancer is well known in the medical field. Treatment techniques, however, vary dramatically depending upon the location of the cancerous tissue and the activity level of the radioactive source used in treatment.
One common treatment procedure involves the use of relatively low activity radioactive seeds. Due to their low activity levels, typically about 1 millicurie/centimeter, these seeds remain resident in, or adjacent to, the tissue undergoing treatment for extended periods of time, for example, several days. As a consequence, the seeds are surgically implanted, thereby allowing the patient to continue normal activities during the resident treatment period.
One of the principal advantages of such low activity treatment procedures is the ease of handling of the radioactive sources or seeds, themselves. While ordinarily stored in radioactive "safes" when not in-use, these low activity seeds may otherwise be handled freely by doctors and support personnel during implantation and removal. The disadvantages of this treatment technique, however, are long residency times and the requirement for surgical implantation and removal, the latter with its attendant trauma to adjacent normal tissue.
At the other end of the treatment spectrum are the high activity radioactive treatment procedures. These procedures, which typically employ radioactive sources in the range of 10 curies, present significant handling and treatment challenges. On the other hand, a significant offsetting advantage of such a treatment regime is its extreme speed. A complete treatment session can be completed in only a few minutes. The patient carries no radioactive implants within him from the treatment center.
A ten curie source cannot be openly handled or exposed to treatment facility doctors and personnel. Even relatively short exposures may result in radiation burns. As a consequence, high activity radiation therapy must be conducted remotely, with the radioactive source being removed from a shielded container or "safe" to the point of treatment, and thereafter returned, all by mechanical means.
It will be appreciated that apparatus for positioning high activity sources must be of uncommon integrity, accuracy, and reliability. It must have safeties, backups, and means for assuring that, in no event, can a source be lost, left behind, misplaced or, simply fail to retract into the safe, even for relatively short durations of time. The possibility for irreversible damage to normal tissue, in the time required for manual intervention upon system failure, is simply too great. As set forth in more detail below, the present invention describes a remote source afterloader having a high degree of reliability and emergency backup protection against system failure or loss of control.
The mechanical placement of high activity sources requires precise and accurate positioning both to assure proper dosage levels to cancerous tissue as well as to minimize damage to adjacent normal tissue. By reason of the intense radiation associated with high activity sources, real-time, hand-guided source placement by the treating physician is precluded. The source, therefore, is inserted through a tube, a needle, or catheter previously surgically positioned in the patient.
The use of catheters, although less invasive than the open surgical implantation of seeds, nevertheless traumatizes tissue along its path of insertion. In delicate tissue regions, for example, in the brain, such trauma must be kept to an absolute minimum. Known prior art high activity sources are affixed to the end of delivery wire of substantial diameter, typically in excess of 1 millimeter. As a consequence, the delivery wire and source must be inserted through correspondingly large tubes, needles or catheters.
Recent developments in high activity source manufacture have resulted in the availability of an ultra-thin iridium source of less than 0.5 millimeters in diameter which, in turn, permits the use of significantly narrower catheters. This source is disclosed in U.S. application Ser. No. 228,400, filed Aug. 4, 1988. In its preferred arrangement, the source comprises a 1 centimeter active region of relatively pure iridium positioned 1 millimeter from the end of 2.1 meter delivery wire. Such ultra-thin radiation sources, in combination with the present remote afterloader, now permit radiation treatment in, or proximate to, delicate tissue areas at heretofore unrealizable low trauma levels.
The present invention, therefore, is directed to a remote afterloader having the capability of properly advancing and positioning ultra-thin wire of 0.5 to 0.75 mm diameter with the utmost reliability and safety. It will be appreciated that these new ultra-thin source wires do not exhibit the same strength characteristics, particularly in buckling, as the more massive prior art wires. Thus, existing remote afterloader apparatus, which were developed for these heavier gauge wires, have proved unsuitable.
One such prior art device, for example, uses a drum onto which the delivery wire is wound, thereby retracting the wire from the catheter and patient. Extension of the wire, however, requires a smooth cylindrical shroud oriented around the outside of the drum against which the wire coil expands as the drum is rotated in the uncoiling direction. Upon contacting the shroud the wire is urged through a narrow opening or slit therein, then, into the catheter for delivery to a tumor site. This arrangement is wholly unsatisfactory for ultra-thin delivery wires. These wires simply do not have sufficient buckling integrity to permit the relatively unguided movement central to drum/shroud operation.
The present afterloader incorporates a dual-capstan drive arrangement in which one capstan positively feeds the delivery wire while the second capstan precisely meters wire movement. Importantly, the path of the delivery wire within the afterloader itself is tightly constrained, in both directions from the capstan drive assembly, thereby precluding buckling of the wire. More specifically, a low friction channel or tube having sufficient length to store all but the active tip region of the delivery wire is provided below the capstan drive. This channel is of minimum cross-section thereby precluding wire bending or deformation. Above the capstan drive, the delivery wire, including the iridium source, feeds into a narrow tubular structure defining the interior of a radioactive safe, then through a narrow outlet channel to a multiple catheter turret assembly. In this manner, there are no open regions within the remote driver apparatus which might permit wire buckling during either extension or retraction.
The above wire containment structure serves another extremely important safety function. It is imperative to establish that the highly radioactive iridium source portion of the delivery wire is, in fact, safely retracted and stored within the safe. Failure to properly identify a non-stored condition could result in a severe overdose to the patient and to personnel who enter the treatment environment under the mistaken belief that the source has been properly retracted.
The present afterloader, by contrast, employs redundant systems to verify proper source storage. One of these systems, importantly, provides unfailing and absolute protection against wire over-retraction. Specifically, the end of the narrow wire channel is obstructed to preclude further wire travel thereby defining a maximum wire retraction limit. This position corresponds to proper stowage of the active region within the lead safe.
Abutting engagement between the delivery wire and channel end does not, however, insure that the active region of the wire has been safely stored. For example, were the delivery wire to sever, the inactive end could properly seat against the channel end while the active region remains outside the safe, possibly still within the patient.
The present afterloader includes a console computer at which an operator can enter a treatment plan for a patient. The plan is checked by the console and high level commands specifying source position within a patient and dosage duration are sent to a remote afterloader computer. The afterloader computer receives and implements the commands by controlling wire movement apparatus. The specific actions of the afterloader as well as its safety and integrity are the responsibility of the afterloader computer.
The present invention provides absolute protection against such false indications of wire storage. In this connection, the wire guide and storage channels additionally serve to facilitate highly accurate wire length measurement. Specifically, a "home" optical wire sensor is precisely placed near the channel outlet to detect the presence or absence of the wire. When a wire is extended, the length of the wire beyond the home optical sensor, as determined from the wire movement metering capstan, is closely monitored by a wire length count maintained in the afterloader computerized controller. Upon retraction of the wire past the home sensor, the wire length count is compared with the wire length count at the home sensor when the wire was first extended. If the retraction count is different from the extension count by more than a threshold value, fault signals are generated to notify operating personnel.
In addition to the absolute and unerring determination of active element storage, it is critical that the position of the active source be known at all times with high accuracy and reliability. Improper positioning now only endangers normal tissue, but may result in the failure to treat cancerous tissue. The remote afterloader control circuitry of the present invention provides a high degree of operational cross-checking with automatic wire retraction upon cross-check failure.
Wire delivery and position determination is predicated upon the previously noted dual-capstan arrangement in which a stepper motor which is controlled by the afterloader computer drives the first capstan and a position encoder, also connected to the afterloader computer, is driven by the second capstan. Each computer controlled step of the drive motor produces a precisely known axial movement of the delivery wire and, in turn, a corresponding and known response from the encoder. The output from the encoder is compared against the stepper motor commands, both on an incremental per step basis and on an overall basis. At the incremental level, the absence of proper encoder signals following one or more steps signifies a wire jam, and further wire delivery is terminated.
The afterloader computer further cross-checks the overall number of encoder output pulses actually received against the number of expected pulses based on the number of stepper motor steps commanded. A predetermined, but small, discrepancy is permitted between the computed and actual number of drive motor steps to account for capstan slippage. However, should encoder outputs cease entirely following stepper motor actuations or should the overall number of encoder outputs not fall within the predetermined limits, it is assumed that a delivery wire jam or obstruction has been encountered. In any event, the precise positioning of the wire cannot be assured under such conditions, and, therefore, the wire will be withdrawn. Withdrawal is first attempted by controlling the stepper motor to withdraw the wire. If the stepper motor fails to satisfactorily withdraw the wire, the stepper motor and wire movement capstan are disengaged and a separate retraction motor is energized to withdraw the wire.
The delivery of high activity radioactive sources requires afterloader apparatus comprising two distinct and separately located subsystems. First, the operator console is provided. This console is located in a room separate from the radioactive source thereby avoiding exposure of treatment personnel to radioactivity while the source is extended from its safe. The second subsystem, the remote afterloader, is the mechanical source storage and delivery apparatus which receives high level commands from the console and physically feeds the active source from the safe t precise locations within the patient, and for precise time intervals.
It is a critical feature and objective of the present invention to position the source accurately within a patient and then to withdraw the source, both steps to be performed with a high degree of certainty that the source is actually where it is supposed to be. As set forth above, the described apparatus provides the requisite accuracy as long as the afterloader computer control is properly functioning.
Computers, however, occasionally malfunction. Therefore, the present afterloader provides for monitoring of proper computer function and, in the event of computer or other malfunction, for the automatic emergency retraction of the radioactive source.
The emergency retraction system functions at the most basic circuit level, thereby virtually eliminating the possibility of emergency backup system failure. In the first instance, the emergency system operates from a constantly recharging backup battery source. This backup source is constantly monitored by the computer which, in turn, signals a backup power failure, simultaneously blocking extension of the active source wire until proper backup system operation has been restored.
The emergency retraction system requires no computer control. It does not utilize the normal capstan drive stepper motors, instead, a separate DC motor driven capstan is provided. Upon primary system failure, power is switched to this motor, thereby forcing full wire retraction. Emergency retraction is timed by a retraction timer. When a retraction has taken longer than a preset time, an audible alarm is sounded to notify operating personnel. This emergency motor continues to operate until the inactive end of the delivery wire engages a switch positioned at the end of the wire storage channel.
Watchdog timers are provided within the remote wire driver subsystem to monitor the afterloader computer. In the event that valid reset signals from the afterloader computer control are not received within a preset interval, computer failure is assumed, and the automatic emergency retraction sequence is engaged. In an embodiment a redundant pair of watchdog timers is used for greater safety. Further, the timers are reset by a multi-bit binary word which follows a predetermined sequence from word to word. A received multi-bit word is compared at the timer with a predicted value and if the received word and the predicted value are not the same, the reset signal is considered invalid. A maximum treatment timer is also used which starts the emergency retraction system when the active source has been extended for more than an expected maximum treatment time.
Additional operational and apparatus subsystems are included to further assure proper overall system operation. One such subsystem is a wire delivery pretest subsystem. This subsystem assures proper active wire extension by first checking the path integrity of each catheter. This test is performed by extending a dummy wire through each catheter tracing the treatment profile intended for the active wire.
The dummy wire drive apparatus is substantially identical to that previously described for the active wire, although no emergency retraction system is incorporated. Thus, undue slippage or jamming of the dummy wire, or a failure to retract fully, signals a fault condition which precludes active wire extension. Importantly, this fault condition is registered, not merely by the computer afterloader, but in hardware interlocks of the remote wire driver apparatus itself, whereby extension of the active wire will be precluded even though the computer may have failed to register the fault condition.
A similar fault detection/protection arrangement is provided in connection with the optional multiple catheter turret. In this connection, the present invention may advantageously incorporate a turret arrangement permitting connection of up to ten separate catheters. In this manner, multiple catheters may be positioned within a patient to facilitate the more complete treatment of the cancerous tissue area in one radiation application session. Under afterloader computer control each catheter is accessed, in turn, and the appropriate pre-programmed treatment regime implemented. This regime includes the above described catheter pretesting by first extending the dummy wire.
It is imperative that no attempt be made to extend the dummy and active wires unless the turret is properly indexed at a valid catheter location having a catheter inserted therein. Consequently, detectors are provided to signal both the existence of the catheter and the proper indexing of the turret. Again, a turret or catheter fault condition is registered, not merely by the computer, but by the remote wire driver apparatus thereby assuring proper fault-induced inaction regardless of computer operation.
From the foregoing it will be apparent that the present invention provides for the control of remotely located radioactive source wire driver equipment. More particularly, apparatus for precisely positioning ultra-thin sources and delivery wires is provided such that the wire may be extended from, and returned to, a safe without likelihood of wire buckling. The proper storage of the active source within the safe is determined with high reliability and the active source is absolutely precluded from over-retraction. A low friction delivery wire channel serves to guide the wire, prevent buckling, preclude over-retraction, and aid in the detection of wire breaks. Emergency backup active wire retraction is provided in the event of computer or other malfunction. Dummy wire testing of all catheters is performed. A multiple catheter selection turret may be provided. Cross-fault detection is employed to preclude active and dummy wire extensions unless the other wire is properly retracted and parked and unless the turret is properly indexed to a valid catheter position. Other features of the invention are disclosed in the following figures, written specification and claims.